1. Field of the Invention
This invention relates in general to a method and apparatus for optimizing the recovery time for a magnetic head, and more particularly, to a method and apparatus for improving baseline recovery of an magneto-resistive (MR) head using a programmable AC coupling pole
2. Description of Related Art
A magneto-resistive head has been conventionally used for reproducing data from a magnetic recording medium. The magneto-resistive element makes use of its physical property wherein the electrical resistance of the magneto-resistive element is changed in response to a sensed intensity of a magnetic field. Thus, the magneto-resistive head may obtain the intensity of the magnetic field from a magnetic recording medium by detecting the change of the electrical resistance.
As mentioned above, a magneto-resistive (MR) transducing head exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the MR element. The value of DC voltage, for a given head, is the product of the constant bias current and the total resistance between the head lead terminals.
However, the prior art reproduction of the recorded data with use of the MR element has the disadvantage that the reproduced waveform by the MR element involves non-linear distortion as the change of the resistance of the MR element to the flowing magnet device is properly non-linear as described above. For example, thermal asperities can locally increase the strip temperature by more than 100 degrees centigrade which may cause a resistance change and consequently a voltage change. Further, thermal fluctuations may produce a resistance change in the elements by passing bias current through them. Accordingly, when a thermal asperity is encountered the operating point of the MR element gets shifted and the data that is being modulated by the head will be unreadable without sufficient error correction processes.
Electronically, this change in the operating point of the MR element causes a baseline shift in the read-back signal. The circuits which are used to amplify the MR head signal will attempt to recover from the baseline shift in order to provide a readable signal. While the circuits are recovering, the data in many cases is not readable and must be recovered by error correction codes or a hard error will occur. The most likely cause of a hard error during a baseline shift is from the amplifiers saturating or from the distortion when the amplifiers are near saturation. To reduce the recovery time from an operating point disturbance and reduce the time the read back signal is saturated in the amplifier, faster recovery time constants can be chosen in the amplifier electronics. The pole that sets the recovery time constant is known as the AC coupling frequency pole of the amplifier or the high pass frequency pole of the amplifier. In the MR pre-amplifier, it can be known as the baseline restore bandwidth.
The problem with setting the AC coupling pole too high is that with the higher pole, data reading problems can occur from too much phase shift and amplitude reduction at the low frequency data patterns.
It can be seen that there is a need for an adjustable AC coupling pole for optimizing the recovery time of a magnetic head.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for improving baseline recovery of an MR head using a programmable AC coupling pole.
The present invention solves the above-described problems by providing a pole with a location that can be chosen for each data band on a file to achieve the fastest recovery from a baseline shift without degrading the normal channel performance. Further, the low frequency pole may be moved during the data recovery procedure to recover from a data read error.
A system in accordance with the principles of the present invention includes a circuit path carrying digital data signals read from a recording media by a magnetic head, an amplifier coupled to the circuit path for amplifying the digital data signals, and a recovery time setting means having an programmable recovery time constant associated therewith. Once a data read error has been detected by the system, the recovery time constant may be adjusted to set a recovery time for the amplifier to provide readable data signals representing the digital data signals read by the magnetic head. In addition, the invention includes a digital controller which is coupled to the recovery time setting means for adjusting the recovery time setting means to change the recovery time constants in response to compensate for the disturbance to the magnetic head.
One aspect of the present invention is that an operational transconductance amplifier having an adjustable gain is coupled to the preamplifier and wherein the gain is selected to set a recovery time for the preamplifier.
Another aspect of the present invention is that the recovery time setting means may be disposed at the output of a magnetic head preamplifier.
Another aspect of the present invention is that the recovery time setting means is a programmable RC filter wherein a resistor value is programmable to set a recovery time for the data channel amplifier.
Yet another aspect of the present invention is that the logic interface is a digital-to-analog converter.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there is illustrated and described specific examples of an apparatus in accordance with the invention.